* The "Story Mode" button on the MainScene opens the levelpacks window.
* Levelpacks from all places are shown (built-in and user files), basic
level picker works.
* When playing a level out of a levelpack: the PlayScene gets the file
data from the zipfile and plays it OK.
* When a levelpack level is solved, the "Next Level" button appears on
the success modal and hitting Return will advance to the next level in
the pack. The final level doesn't show this button.
* The user can edit levelpack levels! Clicking the "Edit" button on the
Play Mode moves the loaded level over to the EditScene and the user
could save it to disk or edit/playtest it perfectly OK! The link to
the levelpack is lost upon opening in the editor, so the "Next Level"
victory button doesn't appear.
* Simplify building Doodle by cloning outside dependencies (like
go/render and go/ui) into the deps/ folder, to assist with Flatpak
building the app from source easily.
